Project Summary/Abstract Staphylococcus aureus (S. aureus) has been strongly implicated as a causal factor in pathogenesis of atopic dermatitis (AD) since the 1970s, and is further supported by publications demonstrating S. aureus skin colonization precedes AD onset. ADRN studies found that ?50% of AD subjects are colonized and this AD phenotype is associated with greater epidermal barrier dysfunction, type 2 immunity, disease severity and remarkable alterations in the skin microbiome (increased S. aureus relative abundance and reduced abundance of the commensal bacteria, Cutibacterium acnes (C. acnes). Additionally, S. aureus-colonized AD subjects are more likely to develop a severe viral condition, called eczema herpeticum. An even more serious condition is eczema vaccinatum, caused by exposure to vaccinia virus (smallpox vaccination), which has a fatality rate as high as 30%. In an effort to explain this, we found discovered that epidermal exposure to a highly virulent S. aureus strain (USA300), commonly found on AD subjects, markedly enhances its susceptibility to vaccinia virus. Collectively, these observations support our hypothesis that S. aureus may be a key driver of disease severity and response to systemic treatments (Aim 1) as well as serious viral complications (Aim 2). Determining what role C. acnes, the most abundant commensal skin bacteria, plays in suppressing S. aureus will be the focus of studies outlined in Aim 3. These studies will leverage the wealth of deeply-phenotyped AD subjects and their biospecimens from previous ADRN studies (ADRN02-Registry, ADRN06-ADEH and ADRN09-Dupilumab) and those enrolled into URMCs Longitudinal ?Real-World? AD Study. The Aims demonstrate how the focus is first on the systemic features of S. aureus colonization (e.g. macro level; Aim 1) and moves to the ?epidermal (host) ? microbiome? interaction (Aim 2) and ultimately to the microbe-microbe interaction at the skin surface (e.g. micro level; Aim 3).